refactor: reduce CollectionService internal method visibility

[adityamparikh]

Summary

• Change buildIndexStats(), buildQueryStats(), getCacheMetrics(), getHandlerMetrics() from public to package-private
• These are internal implementation details that should not be part of the public API surface

Test plan

• ./gradlew build passes
• ./gradlew nativeTest -Pnative passes (119/119 tests)
• No regressions — all existing tests remain in the same package and continue to pass

🤖 Generated with Claude Code

[adityamparikh]

Rebased onto main (a84033b).

Conflict resolution: main had since marked getCacheMetrics/getHandlerMetrics as public @Nullable (they return null when the metrics endpoint is unavailable, e.g. Solr 10). I kept the @Nullable and applied only the visibility reduction, so both are now package-private and still annotated.

I also re-checked that the narrowing is actually safe: every caller of all four methods — buildIndexStats, buildQueryStats, getCacheMetrics, getHandlerMetrics — lives in org.apache.solr.mcp.server.collection, production and test alike (CollectionServiceTest, CollectionServiceIntegrationTest). Nothing outside the package refers to them, so package-private costs no test access.

Verified: ./gradlew build green on JDK 25.